I have had fiddler crabs in the past. They do much better with open air at the top of the tank and will go up there a couple times a day if they have the option. I have also seen them eat fish, and they will do that whenever they can grab on with the big claw and hold on.
You need to assess whether your Cichlids have the swimming power to escape if the crabs latch on. Relative size is probably a big factor. The fish in the picture looks too big to get eaten, but I've been wrong about that before.
The crabs are very entertaining if you can make your tank work for them! They do a teritorrial mating dance of some kind that looks a bit like semaphore. They can, btw
, eat other food, like brine shrimp or even flake food.